Use of optical isolator is obligatory if one of the conditions is met: Optical isolator protects TapHome control unit when connecting Modbus RTU or other RS485 bus device to it. Combined with the right cables, they are going to guarantee the creation of stable and functional RS485 networks.Optical isolator supports bi-directional "half-duplex" transmission for RS485 networks. A pair will be used for "+" and "-" (or "A" and "B"), one of the wires of the second pair will be used as "ground" and the fourth wire will be not-used.Ĭheck out also our article about BIAS and TERMINATION resistors. The cable recommended in this type of connection is the Belden 9842, which has 2 twisted pairs. In this configuration, you can see that the shield is still there and just simply daisy-chained on each cable junction, then connected to ground only on one end like shown here: The ideal RS485 network requires a dedicated wire for ground.Īlso, when some devices on the RS485 are NOT opto-isolated, the third wire becomes essential to avoid the units being damaged. With noisy environments (our sites?) using the shield as ground is not always the most effective solution.Īnd even worse, if you have a non opto-isolated device installed on that network, any noise could "drain" to ground through the device itself instead of flowing through the ground terminal at the end of the line, damaging the device in the process! It gets away most of the time with this configuration, with the shield draining noise and providing the ground reference at the same time. Why? Even without the shield, the RS485 network is quite robust. This works and is sometimes also recommended by many vendors. When the devices on the RS485 network are all opto-isolated, it is common practice to use a 2 wire twisted pair shielded cable, where the shield is used also as "return path" (ground) and is connected to each device like shown here: The circuit reference may be established by a third conductor connecting the common leads of devices, or it may be established by connections in each using equipment to an earth reference." #RS485 OPTICAL ISOLATOR GENERATOR#"Proper operation of the generator and receiver circuits requires the presence of a signal return path between the circuit grounds of the equipment at each end of the interconnection. But these are signal wires and the network always requires a return path. The 2 wires are sometimes referred as "+" and "-" instead of "A" and "B", making the user think that 2 wires is all you need to make it work. #RS485 OPTICAL ISOLATOR PLUS#The RS485 network is wrongly referred to as a "2 wires plus shield" network. The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. No matter the devices, it is always good on real site installation to use a shielded cable, with 18-22AWG wiring and the shield connected ONLY to one end of the line. #RS485 OPTICAL ISOLATOR HOW TO#It is important to know how to wire devices on an RS485 network, especially when a non opto-isolated device is present. Some RS485 devices come with optical isolators installed (also called Galvanic isolation) on the RS485 port, and some don't. How do I wire RS485 devices? A quick schematic guide on how to wire RS485 devices
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